In the prior art, optical pyrometers have been able to calculate the temperature of a remote body by means of measuring the radiation given off by that body and applying the measured numbers to a well known formula. Complications have arisen because the emissivity of the surface and its dependence on wavelength has an effect on the calculation but it is generally not known. Other unknowns include the reflectivity of the surface and the level of extraneous radiation on the surface. The properties of the medium between the surface being measured and the sensor are not known and may vary as a function of wavelength and also as a function of time.
Remote pyrometry is often used in hostile environments, where there is a great deal of spurious radiation from a flame or from hot elements other than the subject of the investigation. In that case, it is difficult or impossible to separate the radiation that is emitted from the body from spurious radiation that is reflected from the body by techniques known in the prior art.
U.S. Pat. No. 4,417,822 illustrates an approach in which a laser irradiates a surface and a detector responsive to a narrow band of radiation employs heterodyne optical detection in an attempt to measure concurrently the radiance and the emissivity of the subject surface. U.S. Pat. No. 3,698,813 illustrates a pyrometer in which the surface is irradiated with modulated light from the remote sensor and the intensity of the reflected light from that surface is measured, thus correcting for the reflection of the remote surface. Both of these devices in the prior art are unable to correct for the properties of the medium between the sensing device and the surface. The use of a narrow spectral range also means that the results depend on the collection optics.